Lecture 2Thursday, September 10th, 2020

Shell Customization, I/O, Job control, Environment Variables, Bash scripting

RecapLast time:

● More Unix commands● Interacting with the shell● File attributes

This time:

● Shell customization● I/O● Job Management● Environment variables● Bash scripting

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Shell Customization

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Shell RecapUsers interact with the shell!

Different types of shells:

● zsh● bash●

Shell

Kernel

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Shell Customization● Each shell supports some customization

○ user prompt settings○ environment variable settings○ aliases

● The customization takes place in startup files, which are read by the shell when it starts up

○ Global files are read first - these are provided by the system administrators (e.g. /etc/profile )

○ Local files are then read in the user’s HOME directory to allow for additional customization

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Shell Startup Files (I)● Different startup files are read depending on which shell you are using● Wikipedia has a nice summary: Shell Configuration Files● The bash shell has two configuration files:

○ ~/.bash_profile - Read at login○ ~/.bashrc - Read at login and when a new window is opened

● The zsh shell uses:○ ~/.zprofile - Read at login○ ~/.zshrc - Read at login and when a new window is opened

● It can sometimes be confusing to keep in mind what all the files do. Here are some refs:

○ Moving to zsh – Scripting OS X

○ What should/shouldn't go in .zshenv, .zshrc, .zlogin, .zprofile, .zlogout?

○ ZSH: .zprofile, .zshrc, .zlogin - What goes where?

○ bash(1) - Linux man page, About bash_profile and bashrc on macOS6

Breakout Room: Shell Startup Files1. Note your breakout room number.2. Figure out who’s birthday is next.3. Create an alias for ls (e.g. ll). Put this in the appropriate startup file!

a. Example: How to Create and Use Alias Command in Linux

4. Change the command line prompt format. Put it in the appropriate startup file!a. Example: How to Change / Set up bash custom prompt (PS1) in Linux

The examples above may use a different shell from yours, but the basic ideas are the same. You will need to figure out where to put the commands.

I like and use this shell theme: https://draculatheme.com/

Windows users may want to check this out if you’re using Git Bash: How do I modify my Git Bash profile in Windows? 7

I/O

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I/O

● File descriptors are associated with each stream○ 0=STDIN 1=STDOUT 2=STDERR

● When a shell runs a program for you:○ Standard input is the keyboard○ Standard output is your screen○ Standard error is your screen

● To end the input, press Ctrl-D on a line; this ends the input stream

ProgramStandard Input(STDIN)

Standard Output(STDOUT)

Standard Error(STDERR)

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Shell Stream Redirection● The shell can attach things other than the keyboard to standard input or

output○ e.g. a file or a pipe

● Use > to tell the shell to store the output of your program in a file○ ls > ls_out

● Use < to tell the shell to get standard input from a file○ sort < nums

● You can combine both forms together!○ sort < nums > sortednums

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Modes of Output Redirection● There are two modes of output redirection

○ > - create mode○ >> - append mode

● ls > foo creates a new file foo, possibly deleting any existing file named foo while ls >> foo appends output to foo

● > only applies to stdout (not stderr)● To redirect stderr to a file, you must specify the request directly

○ 2> redirects stderr (e.g. ls foo 2> err )○ &> redirects stdout and stderr (e.g. ls foo &> /dev/null )○ ls foo > out 2> err redirects stdout to out and stderr to err

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Wildcards● The shell treats some characters as special● These special characters make it easy to specify filenames● * matches anything● Giving the shell * by itself removes * and replaces it with all the filenames in

the current directory● echo prints out whatever you give it (e.g. echo hi prints out hi)● echo * prints out the entire working directory!● ls *.txt lists all files that end with .txt

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Job Control

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Job Control● The shell allows you to manage jobs:

○ Place jobs in the background○ Move a job to the foreground○ Suspend a job○ Kill a job

● Putting an & after a command on the command line will run the job in the background

● Why do this?○ You don’t want to wait for the job to complete○ You can type in a new command right away○ You can have a bunch of jobs running at once

● e.g. ./program > output &

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Job Control: nohup and Terminal Multiplexers● Use nohup if the job will run longer than your session

○ nohup ./program &> output &

● Terminal multiplexers are great for this○ 6 Best terminal multiplexers as of 2020

○ Screen

○ tmux

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Listing JobsThe shell assigns a number to each job

The jobs command lists all background jobs

kill the foreground job using Ctrl-C

Kill a background job using the kill command16

Breakout Room: Practice Listing Jobs1. Use the sleep command to suspend the terminal session for 60 seconds

a. Hint: If you’re never met sleep before, type man sleep at the command line

2. Suspend the job using Ctrl-Z3. Now list the jobs using the jobs command4. The job isn’t in the background; it’s just suspended. Send the job to the

background with bg %n where n is the job id that you obtained from the jobs command

5. Bring the sleep command (your job) back to the foreground with fg %n6. You could let the job finish (since it’s only sleeping for 60 seconds after all), or

you can kill it. Up to you.

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Environment Variables

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Environment Variables● Unix shells maintain a list of environment variables that have a unique name

and value associated with them○ Some of these parameters determine the behavior of the shell○ They also determine which programs get run when commands are entered○ The can provide information about the execution environment to programs

● We can access these variables○ Set new values to customize the shell○ Find out the value to accomplish a task

● Use env to view environment variables● Use echo to print variables

○ echo $PWD○ The $ is needed to access the value of the variable

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PATH● Each time you provide the shell a command to execute, it does the following:

○ Checks to see if the command is a built-in shell command○ If it’s not a built-in shell command, the shell tries to find a program whose name matches the

desired command

● How does the shell know where to look on the filesystem?● The PATH variable tells the shell where to search for programs

● The PATH is a list of directories delimited by colons○ It defines a list and search order○ Directories specified earlier in PATH take precedence○ The search terminates once the matching command is found

● Add more search directories to your path using export:○ export PATH=”$PATH:/Users/dsondak”

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Setting Environment Variables● Setting a Unix environment in bash uses the export command

○ export USE_CUDA=OFF

● Environment variables that you set interactively are only available in your current shell

○ These settings will be lost if you spawn a new shell○ To make more lasting changes, alter the login scripts that affect your particular shell (in bash

this is .bashrc, in zsh this is .zshrc)

● An environment variable can be deleted with the unset command○ unset USE_CUDA

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Unix Scripting

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Unix Scripting● Place all the Unix commands in a file instead of typing them interactively● Useful for automating tasks

○ Repetitive operations on files, etc○ Performing small post-processing operations

● Shells provide basic control syntax for looping, if constructs, etc

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More on Unix Scripting● Shell scripts must begin with a specific line to indicate which shell should be

used to execute the remaining commands in the file○ Use #!/bin/bash in Bash○ Use #!/bin/zsh in Zsh○ These are called shebang lines○ Comment out lines with #

● To run a shell script, it must have execute permission● Excellent resources are available. Here is a recent one:

○ Moving to zsh

○ On the Shebang

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Unix Scripting Permissions

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Unix Scripting: Conditionals

if [ condition_A ]; then # code to run if condition_A trueelif [ condition_B ]; then # code to run if condition_A false and condition_B trueelse # code to run if both conditions falsefi

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Unix Scripting: String Comparisons

Test identity

string1=string2

Test inequality

string1!=string2

The length of string is nonzero

-n stringThe length of string is zero

-z string

You may want to use these in your conditional statements sometimes.

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String Comparisons: Example

#!/bin/zsh

today="monday"if [ "$today" = "monday" ]; then echo "Today is Monday!"fi

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Integer ComparisonsTest identity

int1 -eq int2Test inequality

int1 -ne int2Less than

int1 -lt int2

Greater than

int1 -gt int2Less than or equal

int1 -le int2Greater than or equal

int1 -ge int2

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Integer Comparisons: Example

#!/bin/zsh

x=13y=25if [ $x -lt $y ]; then echo "$x is less than $y"fi

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Common File Tests (I)Sometimes you want to check the state of files and directories:

Test if the file is a directory

-d fileTest if the file is not a directory

-f fileTest if the file has nonzero length

-s file

Test if the file is readable

-r fileTest if the file is writable

-w fileTest if the file is executable

-x file 31

Common File Tests (II)Test if the file is owned by the user

-o file

Test if the file exists

-e file

#!/bin/zsh

foo=$1 # read from command line

if [ -f $foo ]; then echo "$foo exists and is a file."fi 32

RecapToday we covered:

● Shell customization - have fun making your terminal be useful and look great● Redirecting the I/O streams● Controlling jobs: Putting them in the background / foreground, cancelling

them, suspending them● Quick tour through Unix environment variables● Some of the essentials of Unix scripting

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